What is a dew point, and how does it affect my boiler?

You hear it all the time from your local meteorologist: "today's dew point is..." But what exactly is a dew point, and what does it mean for your heating system?

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What is a dew point?

Chances are, you've observed a dew point in action many times, but never thought twice about it. If you've seen condensation on a cold glass on a summer day, or droplets settled across your lawn on an early morning outing, you already know the effects of a dew point. Basically, a dew point is the temperature that air must be cooled to for moisture in the air to turn from vapor to a liquid. That liquid appears as the dew on the grass or the puddle on your coaster (if you're following your mom's rules). However, despite this seemingly simple explanation, the dew point temperature is determined by a lot of factors, including the outdoor temperature, humidity, and air pressure.

Dew point is closely associated with the "relative humidity" or RH, which we're all familiar with seeing listed in our weather apps, or experiencing on a muggy summer day. The RH is the percentage of water vapor in the air at a given time and temperature. For example, an RH of 50% means that the air is holding half of its total capacity for water vapor at the current temperature. What makes RH a bit tricky is that as the temperature falls, air is capable of holding less water, meaning that an RH of 90% at 5ºF and 80ºF would feel vastly different in terms of that sticky, humid feeling you're used to thinking about. Because of this fluctuation, dew point is a more reliable way of measuring humidity.


How does it affect me in real life?

Despite sounding a bit scientific and easy to disregard in daily life, the dew point can actually tell us a lot of things, like if it's a good day to repair and paint that door you've been meaning to get to. As a rule of thumb, you should not paint when the dew point is within a few degrees of the air temperature on warm days, because it will be too damp, and the paint won't dry properly! The excess moisture in the air can also cause wood to expand, causing doors to swell and stick.

Another reason that dew point is important has to do with your boiler. Many modern boilers use condensing technology. For your condensing boiler to work at its highest efficiency and save you the money it promised, it must be able to condense the moisture from the air in the heating system. And this means – you guessed it – the dew point comes into play!


Viessmann Inox-Radial heat exchanger with condensation droplets

How the dew point affects boilers

When a heating system is being designed or a new boiler is being installed, the system designer or installing contractor should pay close attention to the temperature of the water coming back into the boiler – if it's too high, the dew point will not be achieved, and the moisture will remain vaporized and escape up the chimney. However, when the water that flows through your radiators returns to the boiler, if the water temperature is low enough for the dew point to reached, magic happens. The moisture in the air will then be able to condense into water droplets on the boiler's heat exchanger (see a picture of that above), releasing all of that additional energy stored in the form of heat, back into the heating system. This keeps the fuel usage low and the efficiency high!

The exhaust created by your boiler when burning natural gas has a dew point of around 130ºF (55ºC). This means that in order to get condensation to form on the heat exchanger, the water returning to the boiler must be 130ºF or lower. Find a trusted contractor, and ask them to ensure that your boiler is set up to condense properly, so that you get the most out of your heating system. You can also learn more about condensing boilers in this article.


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